1. Technical Field
The invention relates generally to the field of petroleum production and, more particularly, to the enhancement of the recovery of oil from petroleum-bearing formations undergoing secondary and tertiary recovery operations.
2. Background of the Art
In the production of oil from petroleum-bearing reservoirs, it usually is not possible to recover all of the oil in place by primary recovery methods based on the natural forces present in the reservoir. To increase the recovery of oil from these reservoirs a variety of supplemental "secondary" and "tertiary" recovery techniques are customarily employed. The most widely used of these supplemental recovery techniques is water flooding which involves injecting water into the reservoir. As the water moves through the reservoir, it acts to displace the oil therein and carry it to a production system comprising one or more wells from which the oil is recovered.
It has long been recognized that the interfacial tension between the injected water and the reservoir oil, the relative mobilities of the reservoir oil and injected-water, and the wetability characteristics of rock surfaces within the reservoir are important factors influencing the amount of oil potentially recoverable by water flooding. However, where heavy, viscous oils are involved, it may also be necessary to add one or more solvents to "thin out" the oil and lower its viscosity before it is able to move more freely through the reservoir to the production wells. Typical solvents used for this purpose include naphtha, diesel fuel, kerosine, butane and natural gas.
Another widely used water flooding technique involves the addition of one or more surfactants to the flood water to lower the oil-water interfacial tension and/or to alter the wetability characteristics of the reservoir rock. Processes which involve the injection of aqueous surfactant solutions are commonly referred to as surfactant water flooding or as low tension water flooding, the latter term having reference to the mechanism involving the reduction of the oil-water interfacial tension. It has also been proposed to add viscosifiers such as polymeric thickening agents to all or part of the injected water to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and improving the sweep efficiency of the water flood. Among the materials used for this purpose are cationic polyacrylamides, which also help in the displacement of the oil adhering to the rocks in the formation. In still other cases, sulfuric acid may be introduced into oil formations where waxy asphaltenes and maltenes are present as constituents of the oil to form detergent sulfonates therein.
It is also known, as disclosed in U.S. Pat. No. 2,358,665, the teachings of which are incorporated in their entirety by reference, that carbon disulfide can be used as an oil solvent to enhance the recovery of oil in secondary and tertiary oil production operations. Carbon disulfide is an effective solvent for oil since its relatively high density and lack of miscibility with water allow it to achieve close contact with the oil-retaining sands and rocks in the reservoir. However, the use of carbon disulfide for this purpose has been largely discontinued, due to the hazards associated with its extreme volatility, flamability and toxicity.
Accordingly, a significant need exists for a relatively inexpensive, safe, yet effective method for introducing carbon disulfide into a well as a means for enhancing the recovery of residual oil from petroleum wells. The present invention provides thiocarbonate compositions as such a safe and effective means for introducing CS.sub.2 into an oil well to enhance the recovery of oil therefrom.